The DNA repair pathway as a therapeutic target to synergize with trastuzumab deruxtecan in HER2-targeted antibody-drug conjugate–resistant HER2-overexpressing breast cancer

Abstract Background: Anti-HER2 therapies, including the HER2 antibody-drug conjugates trastuzumab emtansine (T-DM1) and trastuzumab deruxtecan (T-DXd), have improved survival outcomes for patients with HER2-overexpressing/amplified (HER2+) metastatic breast cancer. However, intrinsic or acquired resistance to anti-HER2–based therapies remains a clinical challenge for HER2+ metastatic breast cancer, as there is no standard of care following progression on T-DXd. The purpose of this study is to elucidate the mechanisms of resistance to the anti-HER2 antibody-drug conjugate (HER2-directed ADC) and identify targets whose inhibition enhances the efficacy of T-DXd in HER2-directed ADC–refractory HER2+ breast cancer. Methods : Next-generation sequencing was performed to investigate genetic aberrations that arose after anti-HER2 therapy in 10 breast cancer patient samples. We generated T-DM1 and T-DXd–resistant HER2+ breast cancer cell lines. To elucidate their resistance mechanisms and to identify potential synergistic kinase targets for enhancing the efficacy of T-DXd, we used fluorescence in situ hybridization, droplet digital PCR, Western blotting, whole-genome sequencing, cDNA microarray, and synthetic lethal kinome RNA interference screening. In addition, cell viability, colony formation, and xenograft assays were used to determine the synergistic antitumor effect of T-DXd combinations. Results : We found reduced HER2 expression in 4 patients and amplified DNA repair–related genes in 4 patients after anti-HER2 therapy compared to levels before anti-HER2 therapy. We observed reduced ERBB2 gene amplification in all HER2-directed ADC–resistant HER2+ breast cancer cell lines through DNA damage and epigenetic mechanisms. In HER2-directed ADC–resistant HER2+ breast cancer cell lines, pathway analysis identified the DNA repair pathway as a potential target canonical pathway to enhance the efficacy of T-DXd. The combination of T-DXd with ATR inhibitor BAY 1895344 led to significant breast cancer cell death in vitro ( P <0.01) and in vivo ( P <0.01) compared to single agents. Conclusions : The DNA repair pathways may contribute to HER2-directed ADC resistance. Our data justify further exploring the combination of T-DXd with DNA repair–targeting drugs to treat HER2-directed ADC–resistant HER2+ breast cancer.